Changes in intracellular free calcium concentration ([Ca2+]i) represent the most wide-spread and important signaling event for regulating a plethora of cellular responses. Many cell types employ store-operated Ca2+ entry as their principal pathway for Ca2+ influx. This mechanism is engaged following Ca2+ release from stores, where the depleted stores lead to activation of Calcium Release-Activated Ca2+ (CRAC) channels. Recent work has identified stromal interaction molecule (STIM1) and CRAC Modulator 1 (CRACM1 or Orai1) as essential components for functional store operated Ca2+ entry. STIM1 and CRACM1 are sufficient to reconstitute and amplify CRAC currents in heterologous expression systems. In mammals, there exist several homologs of these proteins: STIM1 and STIM2 in the endoplasmic reticulum and CRACM1, CRACM2, and CRACM3 in the plasma membrane. The role of STIM2 in store operated Ca2+ entry appears to be complex and remains incompletely understood.
A need exists, therefore, for a method by which the level and role of STIM2 on the activity of CRAC channels can be assessed.